1. Field of the Invention
The present invention relates to a charge transfer apparatus which reduces the detection capacity of a signal charge detector and enhances the detection sensitivity. Particularly, there is provided a signal charge detector which suppresses the threshold value dispersion of an output electrode and a reset electrode and has stable characteristics.
2. Description of the Related Art
A charge transfer apparatus which accumulates and transfers information such as incident light and electric signal in the form of electric charges and extracts the information as a voltage signal has been broadly used for applications such as a solid image pickup apparatus in recent years. The solid image pickup apparatus is constituted of a photoelectric converter for converting light to an electric signal (signal charge), a charge transfer section for transferring the converted signal charge, and a signal charge detector (signal output section) for detecting the signal charge transferred from the charge transfer section. A charge coupled device (hereinafter abbreviated as CCD) is used as the charge transfer section. To detect the signal charge, a floating diffusion type signal charge detector is usually used.
FIG. 1 is a sectional view showing an interline transfer system CCD type solid image pickup apparatus disclosed in Japanese Patent Application Laid-Open No. 218104/1993.
Formed on a P-type semiconductor substrate 1 are an N-type impurity diffusion layer 2 as an embedded channel region, a device separating P-type impurity diffusion layer 3, a device separating oxide film 4a, and a gate insulation film 5. A first gate electrode 6 formed of polycrystalline silicon as a transfer electrode, and a second gate electrode 8 formed of polycrystalline silicon are formed on the gate insulation film 5. An insulation film 7 is formed on the surface of the first gate electrode 6, and a potential barrier P-type diffusion layer 9 is formed on the semiconductor substrate under the second gate electrode 8. Each first gate electrode of the charge transfer section is electrically connected to the second gate electrode on the left side. Clocks .phi.1, .phi.2 are alternately applied to the connection part of the first and second gate electrodes.
A fixed output gate voltage V2 is applied to an output gate electrode 10. An N-type floating diffusion layer 11 as the embedded channel region for detecting a transfer charge amount is formed in the same process as that of the N-type impurity diffusion layer 2. The potential change of the N-type floating diffusion layer 11 is detected by an output transistor 14. A reset drain voltage V1 is applied to an N-type drain diffusion layer 12. The potential of the N-type floating diffusion layer 11 is periodically set to the reset drain voltage V1 by a reset gate electrode 13.
The operation of this charge transfer apparatus will be described. First, by applying a reset pulse to the reset gate electrode 13, the potential of the N-type floating diffusion layer 11 is reset to the reset drain voltage V1. In this case, the clock .phi.1 has a high potential, the clock .phi.2 has a low potential, and the electric charge is accumulated under the first gate electrode to which .phi.1 is applied. Subsequently, when the clock .phi.1 is set to the low potential, and the clock .phi.2 is set to the high potential, the electric charge under the final gate electrode (first gate electrode) with the clock .phi.1 applied thereto flows into the N-type floating diffusion layer 11 through a channel under the output gate electrode 10. As a result, a potential change is generated in the N-type floating diffusion layer 11, and the potential change is detected by the output transistor 14.
When the charge amount of the signal charge transferred to the N-type floating diffusion layer 11 is Q, and the capacity of the N-type floating diffusion layer 11 is C, the potential change .DELTA.V of the N-type floating diffusion layer 11 before and after the flow-in of the electric charge is as follows: EQU .DELTA.V=Q/C.
The magnitude of the output signal to the signal charge amount Q increases as the capacity C of the floating diffusion layer 11 decreases. Specifically, as the capacity C of the floating diffusion layer 11 decreases, the detection sensitivity defined by the output voltage to the constant signal charge amount Q rises.
Therefore, in order to enhance the detection sensitivity, in the charge transfer apparatus disclosed in the Japanese Patent Application Laid-Open No. 218104/1993, an insulation film 4b on the surface of the N-type floating diffusion layer 11 is formed in the same process as that of the device separating insulation film 4a. By disposing the insulation film 4b thicker than the gate insulation film 5 in this manner, the capacity of the floating diffusion layer is decreased, and the detection sensitivity of the signal charge is enhanced.
However, the reduction of the capacity C is insufficient in this charge transfer apparatus.
Moreover, in the charge transfer apparatus, the insulation film 4b on the surface of the N-type floating diffusion layer 11 is formed in the same process as that of the device separating insulation film 4a. In this case, however, three mask formation processes are necessary for the ion injection for forming the P-type impurity diffusion layer 3 under the device separating insulation film 4a, the ion injection for forming the N-type floating diffusion layer 11, and the formation of the insulation films 4a, 4b on the P-type impurity diffusion layer 3 and N-type floating diffusion layer 11. It is difficult to match the masks, and there is a problem that the apparatus provided with a desired performance cannot be formed.